As a security technique for the Internet, IPSec (IP Security) is known. With IPSec, it is necessary to implement the IKE (Internet Key Exchange) protocol to realize a safe key exchange. However, IKE requires communication apparatuses for carrying out communication to hold setting information individually, so that the operation becomes complicated. Here, a conventional scheme is proposed where an simplified key exchange is performed for enabling smooth operation by distributing the keys to the specified apparatus (see, Patent Document 1).
In the communication system where this conventional key exchanging scheme is adopted, an expiration time is assigned to the key in order to ensure communication security. As the expiration time approaches, the communication apparatus which is communicating acquires a new key by requesting the distribution of a new key to the key distributing apparatus, and updates the key.    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-292135